1. Field of the Invention
The present invention relates to magnetic recording media used in hard disk drives (HDDs), and more particularly, to a perpendicular magnetic recording medium with improved thermal stability of record information and high signal-to-noise ratio.
2. Description of the Related Art
In longitudinal magnetic recording (LMR) applied to hard disk drives (HDDs), a major external data storage device of computers, the size of a data record domain in a magnetic disk has decreased with microstructure as the need for high-density data recording increases. However, this decrease in size makes the data record domains susceptible to removal by thermal energy generated by operation of the HDD which is more dominant than magnetostatic energy from the data record domain. This is referred to as the super paramagnetic effect. To overcome the super paramagnetic effect, the LMR technique has been replaced by a perpendicular magnetic recording (PMR) technique for HDD applications. The PMR technique uses a higher electrostatic energy and lower demagnetization energy compared to the LMR technique, so it is advantageous in high-density data recording. The high-density PMR technique also has enabled detection of a micro data domain in combination with advances in the manufacture of highly sensitive read heads.
The schematic structure of a single-layer PMR medium is shown in FIG. 1. The single-layer PMR medium includes an underlayer 12 for promoting the perpendicular orientation of a perpendicular magnetic recording layer 13 formed over the underlayer 12, the perpendicular magnetic recording layer 13 having the perpendicular magnetic anisotropy energy to keep the perpendicular orientation of the data record domain, a protective layer 14 for protecting the perpendicular magnetic recording layer 13 from external impacts, and a lubricant layer 15 on a glass or aluminum alloy substrate 11.
The perpendicular magnetic recording layer 13 has the perpendicular magnetic anisotropy energy with a magnetic easy axis oriented perpendicular to the plane of the perpendicular magnetic recording layer 13 due to the underlayer 12.
Recording density in a perpendicular magnetic recording mechanism is largely affected by the characteristics of the perpendicular magnetic recording layer and the perpendicular orientation promoting underlayer.
In a conventional PMR medium, as shown in FIG. 1A, an underlayer disposed below a PMR layer to promote perpendicular magnetic orientation of the PMR layer is formed of titanium (Ti), and the PMR layer is formed of a Co alloy. In this case, the mismatch between the lattice constants of the Ti underlayer and the Co-alloy PMR layer is considerably high at about 18%. When the lattice constant mismatch becomes larger, a perpendicular magnetic degradation layer 16, a buffer layer with inferior magnetic properties, is formed on the Ti underlayer in the course of crystal growth of the Co-ally PMR layer, as shown in FIG. 1B. The perpendicular magnetization degradation layer 16 reduces the thickness of the crystalline PMR layer. As a result, the magnetic properties of the PMR layer are degraded with low signal-to-noise (SNR) ratio.
A Ti layer typically used for the perpendicular orientation promoting layer shows poor perpendicular orientation properties when deposited because the value of Δθ50 is large, which indicates an angle of crystal growth deviating from the perpendicular axis. Such a poor degree of the perpendicular orientation in the underlayer affects the perpendicular orientation of crystals being grown for the PMR layer and thereby increases Δθ50 of the PMR layer.
An increase in grain size of the PMR layer as well as the perpendicular orientation reduction described above reduces the SNR of the PMR layer, thereby limiting high-density recording.